Optical encoders are roughly grouped into transmissive optical encoders using light that transmits through a movable member formed with slits and including a light transmissive portion and a non-transmissive portion, and reflective optical encoders using light reflected by a movable member formed with slits and including a light reflective portion and a non-reflective portion. Among the transmissive and reflective optical encoders, some optical encoders use a stationary member formed with slits and including a light transmissive portion and a non-transmissive portion that are disposed such that light incident on a light receiving element disposed to face the movable member varies in accordance with displacement of the movable member. Other optical encoders do not use a stationary member formed with slit, but include a plurality of light receiving elements disposed in such an arrangement pattern that the light receiving elements serve as a stationary member. In optical encoder devices, a signal is generated in accordance with the amount of transmitted light or reflected light determined by the relationship between the pattern of the slits in the movable member and the pattern of the slits in the stationary member or the pattern of the light receiving elements. The generated signal is multiplied to obtain a signal with a higher resolution. In order to obtain a high-precision signal, it is desirable that the original signal to be multiplied should have a sinusoidal waveform. However, the slits in the stationary member or the light receiving elements according to the related art are disposed at intervals equal to the pitch or cycle P of the slits in the movable member. The signal generated with such a pattern of the slits in the stationary member or the light receiving elements theoretically has a triangular waveform which involves significant distortion. Such distortion is caused by high-order components, rather than a fundamental-wave component. A third harmonic wave has an amplitude of about 11%, a fifth harmonic wave has an amplitude of about 4%, and a seventh harmonic wave has an amplitude of about 2%, with respect to the fundamental-wave component. The distortion rate represented by the root mean square of the ratio of the harmonic-wave components (for example, calculated up to the 27th harmonic wave) to the fundamental-wave component is as high as 12.11%.
In order to address this issue, various attempts have been made on the intervals between the slits in the stationary member or the plurality of light receiving elements. According to Japanese Patent Application Publication No. 60-42616 (JP60-42616A), a phase difference of 1/6 is provided between two slits to cancel a third harmonic wave.
According to Japanese Patent No. 2539269 (JP2539269), a phase difference of 1/6 or 1/10 is provided between two slits or slit groups to cancel a third or fifth harmonic wave.
Japanese Patent No. 3184419 (JP3184419) improves the technique disclosed in JP2539269, and 2′ or more slits are disposed to cancel x orders of harmonic waves.
According to Japanese Patent Application Publication No. 2007-218603 (JP2007-218603A), a phase difference of 1/12 is provided between four slits to cancel a third harmonic wave.
JP2007-218603A teaches a known example of an optical encoder device that provides an output signal with little distortion. In the known optical encoder device, slits provided in a stationary slit plate are divided into units having a phase difference of 1/12 of the pitch P of slits in a movable slit plate and each including light transmissive slits of which the number is a multiple of four. Defining the phase of a first unit of the light transmissive slits as a reference, the first and second units of the light transmissive slits are shifted in phase by P/12, the second and third units of the light transmissive slits are shifted in phase by P/6, and the third and fourth units of the light transmissive slits are shifted in phase by P/4.
JP60-42616A teaches an optical encoder in which a phase difference of 1/6 is provided between two slits to cancel a third harmonic wave.
JP2539269 teaches an optical encoder in which a phase difference of 1/6 or 1/10 is provided between two slits or slit groups to cancel a third harmonic wave or a fifth harmonic wave.
JP3184419 describes that x orders of harmonic waves are canceled by using 2x or more slits.
According to the techniques disclosed in JP60-42616A, JP2539269, and JP2007-218603A, the distortion rate is significantly reduced to 4.63%, 1.59%, and 1.60%, respectively, with respect to the original waveforms. The obtained waveform, however, still involves distortion with a distortion rate of 1.55% or higher.
In designing optical encoders, the number of slits in a stationary member or the number of light receiving elements is determined to provide a desired number of pulses in consideration of constraints on the length of the light receiving element pattern, the length of the light receiving elements, and the gap between the light receiving elements, the intensity of the light source, the diameter (for rotary optical encoders), and so forth.
According to the technique disclosed in JP3184419, the distortion rate is reduced to 0.79% when the number of slits is eight. In order to further reduce the distortion rate, however, it is necessary to increase the number of slits to 16, 32, and so forth by multiplying the number by 2, which makes it difficult to develop a design that effectively uses the length of the light receiving pattern and so forth discussed above.